1. Field of the Invention
The field of the invention is fluidized catalytic cracking of heavy hydrocarbon feeds and cyclones for separating fine solids from vapor streams.
2. Description of Related Art
Catalytic cracking is widely used to convert heavy feed into lighter products by catalytically cracking large molecules into smaller molecules. In fluidized catalytic cracking (FCC) catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalyst and fines must also be removed from flue gas discharged from the regenerator. Any catalyst not recovered by the regenerator cyclones stays with the flue gas, unless an electrostatic precipitator, bag house, or some sort of removal stage is added at considerable cost. The amount of fines in most FCC flue gas streams exiting the regenerator is enough to cause severe erosion of turbine blades if a power recovery system is installed to try to recover some of the energy in the regenerator flue gas stream.
The solids remaining at this point are difficult to recover, having successfully passed through several stages of highly efficient cyclones. The solids are small, essentially all are below 20 microns, and include significant amounts of submicron to under 5 micron sized material.
Collection of such solids has been a challenge for almost a century. A survey of the state of the art is described in Perry's Chemical Engineering Handbook, in DUST-COLLECTION EQUIPMENT, discussed in the parent application. FCC operators typically use cyclones to separate solids from gas.
Refiners typically use 2 to 8 primary and 2 to 8 secondary cyclones in their FCC regenerators, because of mechanical constraints and pressure drop concerns. These cyclones have a fairly large diameter, which restricts the amount of centrifugal acceleration which can be achieved.
Thus FCC regenerators inherently let a large amount of fines and dust, in the below 15 micron range, pass out with the flue gas. This material must be removed from the flue gas prior to discharge to the atmosphere, or passage through a power recovery turbine.
Generally a third stage separator is installed upstream of the turbine to reduce the catalyst loading and protect the turbine blades, or permit discharge of flue gas to the air. These can be 20, 50, 100 or even more small diameter cyclones. The third stage separator can use large numbers of small cyclones because it is not in or a part of the FCC regenerator. Small diameter cyclones are used because these give better fines collection than larger cyclones, for the same gas velocity and pressure drop. Perry's Chemical Engineer's Handbook, Sixth Edition, in Table 20-33 reports that for a 5-20 micron dust mixture, dust collection improves significantly as cyclone diameter decreased, with efficiencies for 6, 9 and 24 inch cyclones being 90%, 83% and 70% respectively.
Several vendors (Polutrol and Emtrol) supply systems with many small diameter, horizontally mounted, closely connected and radially distributed cyclones about a central gas outlet. The use of multiple, small, horizontally mounted cyclones is also known for general dust removal, see e.g., the Dustex miniature collector assemble shown in FIG. 20-108 of Perry's Chemical Engineering Handbook, Sixth Edition. Gas is tangentially added to a great number of generally horizontally mounted cyclones. Purified gas is withdrawn via a central gas outlet near the tangential inlet, while dust is removed from the opposite end of the cyclone, which may be of reduced diameter but is unsealed.
Our parent application was on improving the operation of horizontal cyclones. The problem was dust from an upper horizontal cyclone falling past a lower horizontal cyclone, with some dust reentrained in the vortex associated with a lower cyclone. Adding a "scroll" or "half-pipe" addition to the cyclone barrel improved solids recovery. The vortex associated with the cyclone was either enclosed (scroll) or protected somewhat (half-pipe extension).
While the improvements achieved in our parent case were significant, they did not address the different problems associated with third stage separators using large numbers of vertical cyclones. Recovery efficiency in these units was also not as high as desired.
We studied vertical third stage separators and believed that the problem was that the cyclone vortex extended out the bottom of the cyclones. This vortex extended into the collection chamber and was able to capture particles from the collection chamber and "emit" such particles.
We discovered that adding a device to shield the collected particles from the vortex improved collection efficiency.